Auditory warning systems for human interfaces are often designed around criteria that depend primarily upon signal loudness. It is well understood from the auditory literature that, by making an alert signal substantially louder than the measured background noise level, one can insure that an alert signal will be detectable. For example, an ISO standard 7731 (“Danger signals for work places—Auditory danger signals”, ISO Standard 7731-1986(E)) specifies that an auditory alert signal be issued with frequency components at a sound pressure level at least 13 dB above an average level of all background sounds. This approach to detection is referred to as “exceeding the masked threshold”; the spectral components of the alert signal have sufficient amplitude so that these components can be heard. As used herein, “noise” refers to non-information-bearing auditory signals, and “background sound” includes noise and information-bearing auditory signals whose content is not of interest for the task at hand (e.g., for purposes of distinguishing presence of an auditory alert signal). Usually, but not always, the noise level or background sound level has been time averaged over a time interval of appropriate length.
For a typical design of an auditory alert system, the overall amplitude or sound pressure level is often set at a value substantially greater than the background sound level. This approach is simple to understand and to implement. However, if an alert signal sound pressure level is too loud, the alert signal may produce a “startle effect” that hinders performance in some high stress situations. High amplitude alarms have been used in the past because (1) most communication equipment was of limited audio fidelity and (2) loudspeakers, located at a substantial distance form the subject, or monaural (single ear) auditory signal systems, were used for such communications.
What is needed is an alternative approach that uses other features, such as frequency component processing and/or spatial modulation of signals, to improve the detectability of an alert signal, without substantially increasing the amplitude level of an alert signal beyond the background sound level. The approach should preferably be able to combine acoustical features, other than amplitude, to provide greater improvements in alert signal detectability. Ideally, but not necessarily, an alert signal is delivered to a subject through two stereo earphones.